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| Home > Publications database > Kleinwinkelstreuung mit Neutronen an Typ-II Supraleitern und Polymeren |
| Book/Report | FZJ-2018-00959 |
1977
Kernforschungsanlage Jülich, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/16900
Report No.: Juel-1385
Abstract: This report on "small angle neutron scattering from type II superconductors and polymers" is divided into three parts concerning (I) the technique of small angle neutron scattering (SANS), (lI) the type II superconductivity investigated by neutron diffraction from flux line lattices, and (III) the configuration of polymer molecules investigated by neutron scattering from polymer molecules which were tagged by deuteration. (I) The differential cross section of nuclear and magnetic elastic SANS is presented and discussed when specialized to the case of a two phase system. The scattered neutron intensity for a given scattering cross section and a given instrument is calculated and several parameters are varied with the purpose to maximize the scattering intensity. This optimization leads to a concept how to construct a SANS instrument. According to this concept the instrument at the FRJ-2 in Jülich was constructed as is shown in a description of that instrument. Comparing an optimized small angle neutron scattering experiment with a similar x-ray experiment having the same resolution it is demonstrated that despite the extremely small luminosity of a neutron source the scattered intensity is often even higher for neutrons than for x-rays. Further favourable properties of SANS experiments are discussed by comparing them with x-ray experiments. These properties are based on the use of long wavelength neutrons, on the irregular dependence of the nuclear scattering length on the atomic number, and on the existence of a magnetic neutron interaction. (II) For the neutron diffraction from flux lines in type II superconductors the integral reflectivity is calculated. Using experimentally determined reflectivities it is shown how to determine the phase factors in order to evaluate microscopic field distributions within a unit cell of the flux line lattice from the integrated reflectivities. Such experimentally determined microscopic field distributions in pure Nb, NbTa, Pb-Bi and NbN are compared with those calculated from various theories. This comparison shows that the agreement of the experimental results is reasonable good with Kramer and Pesch's numerical results computed from the Eilenberger equations and with Brandt's approximate solutions of the Gorkov equations. The diffraction experiments with flux line lattices yielded a large amount of information concerning the morphology of a flux line lattice with various external parameters. The morphology of vortex lattices influenced by temperature, flux density, magnetic prehistory of the superconductor, its shape, its pinning strength, transport current and the way to produce flux line lattices is reviewed. For single crystalline specimens the following phenomena were observed: The vortex lattices are single crystalline. There are correlations between crystallographic directions of the crystal and vortex lattice. The crystal symmetry of the field axis influences the symmetry of the vortex lattice. These results were obtained in Nb, PbBi, PbTl and Tc. Various experiments showed that these phenomena are hardly influenced by the parameters mentioned &bove. However, the quality of the flux line lattice is easily influenced by almost all of the parameters and thepinning has the strongest effect. The correlations are described by two different theoretical attempts based according to Takanaka on the anisotropy of the Fermi surface and according to Ullmaier on the anisotropie magnetoelastic interaction between crystal and flux line lattice. Forces exerted to flux lines can cause viscous motion of the flux lines and a bending of the vortices. Aberration occurs when a neutron beam is diffracted from a moving flux line lattice. This effect was used to determine the flux line velocity coupled with a transport current which exceeds the critical one. The bending of flux lines caused by basic interaction forces between flux lines and pinning centers was deduced from rocking curves in order to determine more directly the pinning force than is possible by any other method. (111) Various configuration models had been developed to describe the polymer configuration in the aroorphous and crystalline state. An experimental investigation of the moleeule configuration in the solid became possible by SANS from molecules which are tagged by deuteration. With this technique it was shown for amorphous polystyrene and polymethylmetacrylate that the coiling is identical to that in B-solvents as was predicted by Flory. In the investigation of partially crystallized polyethylene problems due to void scattering and clustering of the tagged moleeules had to be solved.
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